Conglomerate and breccia (rudite)
Rudites are coarse-grained clastic (terrigenous) sedimentary rocks consisting of ruditic clasts > 2 mm diameter (gravel: pebble, cobbles, and boulders) surrounded by fine-grained material consisting of sand, mud, and and crystalline cement. The term ‘rudite’ derives from the Latin rudus, meaning ‘debris’ or ‘rubble’, a term proposed by Grabau that is a synonym of psephite (same meaning, but with a Greek root). Rudites comprise two-rock types, conglomerate and breccia, that are characterized by the same grain size and differ for the roundness of the clasts.
- Conglomerate show rounded to sub-angular ruditic clasts that have been transported by a current that smoothed their edges. Conglomerates are produced by fluvial systems (alluvial plains, alluvial fans, deltas…), wave action (beach deposits), but also some gravity flows (debris flows, turbidity currents).
- Breccia consists of angular or very angular clasts with sharp edges (rubble) that have not been smoothed by a selective current. In sedimentary environments, breccias are the result of deposition in mass by processes that disintegrate the parent material of the clasts (e.g. landslides, rock avalanches, grain flow, sinkholes…). There are also several, other (i.e. non sedimentary) processes that are able to crush rock formations producing angular clasts, like meteor impacts (impact breccia), volcanic eruptions (volcanic breccia), hydrothermal activity (hydrothermal breccias), fault slip (fault breccia), just to cite some. These other breccia types are treated in the corresponding igneous/metamorphic sections.
Conglomerate and breccia may comprise a wide range of clasts with extremely different grain size, ranging from granules (2-4 mm) to boulders (> 25,6 cm), and variable composition (e.g. clasts of metamorphic, igneous or sedimentary rocks). Therefore, more precise descriptive names should incorporate at least the dominant grain size and clast lithology in the name of the rock (e.g. granite-pebble conglomerate; quartz-cobble breccia).
The coarser (ruditic) clasts of conglomerate and breccia predominantly consist of fragments of other rocks (lithic fragments) that internally preserve the texture of the parent material. Single mineral (monomineralic) clasts of ruditic grain size are uncommon because only a few igneous and metamorphic rocks are coarse enough to produce pebble-sized single mineral grains. The coarse-grained framework grains are generally surrounded by a fine-grained matrix consisting of sand, silt and/or clay (mud) derived from the disintegration and alteration of rocks. Quartz tends to be the most abundant constituent of the grains and the matrix because of its resistance to erosion, followed by feldspars. The clasts and the matrix are cemented together by siliceous, carbonatic, or iron oxide-rich cement precipitated after deposition in the pore spaces between grains. Some well-sorted conglomerates (e.g. orthoconglomerates, see below) may contain mostly cement between the ruditic grains and minor amounts of sand, silt, and clay.
Like sandstones, conglomerates and breccias can also be described in terms of grain shape, sorting, roundness, and packing. In general, the sorting of rudites is poor, because many grain sizes (e.g. gravel clasts and sandy matrix) occur together. The shape of clasts generally reflects the composition of lithic fragments: schistose metamorphic rocks tend to develop flatter clasts than more isotropic rocks, like granites and marbles, that generally develop more spheroidal, equi-dimensional shapes. Experiments show that clasts tend to become rounded (or even well-rounded) after some tens of kilometers of river transport (Prothero & Schwab, 2013). Some cobbles and pebbles may preserve grain surface features such as striations, percussion marks or surface polish. Striations are typically produced by glacial transport. Wind erosion can also produce distinctive ventifacts or surface polish gloss.
Classification of conglomerate and breccia
The first-order classification of conglomerate and breccia (based on Pettijohn, 1975 and modified by Boggs, 1992) is based on the proportion of framework grains (by definition all clasts > 2 mm) and the surrounding matrix. Orthoconglomerates (literally ‘true’ conglomerates) contain more than 85% ruditic clasts and less than 15% matrix. Orthoconglomerates are always grain-supported and the clasts show a prevalence of long (tangential), concave-convex, and sutured contacts (see packing). Conglomerates containing more than 15% matrix are called paraconglomerates. Paraconglomerates can be grain-supported (matrix between 15 and 50% by volume) or matrix-supported (matrix > 50%), characterized by clasts that are not in contact with each other. Matrix-supported paraconglomerates are also called diamictite or diamixtite. The sandy/muddy matrix of paraconglomerates can be laminated, due to currents that were active during deposition. The presence of such an important structure should be specified in the name (e.g. laminated paraconglomerate). The distinction between orthoconglomerate and paraconglomerate is important, because these rocks form in very different sedimentary environments. The formation of orthoconglomerates requires a strong selective current able to remove the finer sediment, whereas paraconglomerates are produced by less selective transport and deposition mechanisms (e.g. debris flow).
The classification of breccias is based on the matrix/clasts ratios of conglomerates: orthobreccia (matrix < 15%) and parabreccia (matrix > 15%). Breccias, however, are not generally produced by selective transport and it is more relevant to classify them based on the composition of the clasts (see below).
Classification based on the composition of the clasts
The classification of conglomerates and breccias can be refined by analyzing the composition of the ruditic clasts (framework grains).
- Oligomictic (or oligomict) rudites consist of fragments of a few resistant rocks and minerals. The extreme case is represented by monomictic (monomict) conglomerate and breccia, which contain fragments of a single mineral or rock.
- Polymictic (or polymict) rudites contain a variety of clasts of different rocks and minerals. If many metastable/unstable rocks and minerals are present (e.g. basalt, slate, limestone) the conglomerate/breccia is termed petromictic (or petromict).
This classification is useful because it allows to understand how long the components of a rudite have been transported. A petromictic conglomerate, rich in unstable rocks, likely experienced a very limited transport compared to an oligomictic conglomerate, where only few resistant minerals (e.g. quartz and feldspars) survived. Oligomictic and monomictic conglomerate/breccia may also form because only one or few rock types are available in the source area, for example a scarp breccia sourced by a single formation.
Intraformational and extraformational rudites
Rudites can be classified based on the source of the clasts. Intraformational conglomerates/breccias contain clasts that derive from the same sedimentary formation which they are part of. In intraformational rudites the clasts have the same composition of the matrix that surround them and of the other sedimentary rocks present in the formation where they are found. These rocks are produced by events of brecciation or clastic reworking that interrupt the normal sedimentation of a basin, for example a storm or an episode of emersion. In general such events produce intraformational shale or limestone pebbles (e.g. rip-up clasts torn off the bottom of a basin by a current).
Extraformational conglomerates and breccias consist of clasts sourced from outside of the sedimentary basin where they are deposited. In this case (which is the dominant situation in sedimentary rocks), the framework grains differ in composition from the matrix. Typical extraformational conglomerates contain fragments of igneous, metamorphic and sedimentary rocks of different age derived from the disintegration, weathering, and erosion of different rock types.
Classification of conglomerates and breccias based on the composition of the matrix
Beyond the well-accepted ortho- and para- classification above, there are many other schemes that classify gravel-bearing siliciclastic rocks based on the composition of the matrix between the clasts. One of the most widely used is the gravel-sand-mud diagram by Folk (1980), shown below (slide to see the corresponding sediment and rock names).
The difference between the ortho-/para- conglomerate/breccia classification and this kind of schemes is that the former provides information about the texture (little or no matrix, grain-supported, matrix-supported), while the latter emphasizes what matrix is present between gravel clasts (e.g. sandy mud, sand, etc.). The ortho-/para- classification (and all the specifiers like the -mict terms) are more useful for sedimentologists, whereas gravel-sand-mud diagrams like the one above provide more information on the composition of the terrigenous mixture and are hence more useful for geotechnics/applied geology. In any case, it is possible to improve the ortho-/para- classification specifying the composition of the matrix (e.g. grain-supported paraconglomerate with sandy matrix).
Examples of conglomerate
Examples of breccia